Millionaires are twice as likely to keep working in retirement than the broader population, according to a new survey from Merrill Lynch and Age Wave.

A third of the people polled with $1 million to $5 million in investible assets are currently working in retirement. That compares with just 15 percent of those retirees with less than $250,000 who are still working.

What's the point of working if you're rich enough to retire? Because they enjoy it. Nearly all of the affluent retirees surveyed said they keep working because "they want to," according to the study.

Today's boomer retirees are climbers and strivers, and sitting on a beach or playing shuffleboard can only satisfy their soul for so long. Everyone needs a game to play-even when you've already won.

Wealthy retirees cite "staying mentally active" as the top specific reason they continue to work, according to the study. They cited that response six times more than "for the money." They also said their work in retirement keeps them more youthful, fit and mentally sharp.

But there are other reasons the rich keep working. More than half of affluent retirees work for themselves and own their own businesses. Being able to set your own hours, your own pay, your own office and your own direction is a lot more enjoyable than bagging groceries or reporting to a cubicle every morning.

Their definition of "work" is also a bit more flexible. About half of wealthy retirees took a break from work when they first retired. The study calls this a "career intermission." It's a chance to recharge, relax and find a new career.

That new career may not be as demanding as their last. Half of wealthy retirees have transitioned to new lines of work that give them more "flexibility," "fun" and "fulfillment."

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